1. Field of the Invention
This invention relates to an undercoating composition for polyesters, and, more particularly, it is concerned with an undercoating composition for polyesters for photographic use, which undercoating contains a compound having two or more epoxy groups in its molecule and which is capable of giving strong adhesion to photographic emulsion layers, as well as a photographic material comprising the same.
2. Description of the Prior Art
Polyester films are more excellent in mechanical properties, dimensional stability, heat resistance, chemical resistance and transparency than other plastic supports, and, therefore, their use, for example, in photographic materials, printing materials, drawing materials, magnetic materials, etc., has lately increased. In particular, polyester films have lately come into great demand as supports for photographic films.
As is well known in the art, various undercoating methods are used to obtain strong adhesion between a polyester film and a photographic emulsion layer. However, polyesters are generally too hydrophobic to show good adhesiveness to a hydrophilic photographic emulsion layer, due to their high crystallinity, chemical inactivity, their high chemical resistance (proper solvents for polyesters are difficult to find) and their having no hydrophilic groups.
To the present, many efforts have been made to solve this problem, for example: (1) subjecting a polyester to a surface treatment such as chemical treatment, mechanical treatment, discharge treatment, flame treatment, ultraviolet ray treatment, high frequency treatment, glow discharge treatment, plasma treatment, razor treatment, mixed acid treatment or ozone oxidation treatment and then coating the same with a photographic emulsion; and (2) subjecting a polyester to a surface treatment as described above, coating the same with an undercoating layer and then with a photographic emulsion, as disclosed in U.S. Pat. Nos. 2,698,241; 2,764,520; 2,934,937; 3,072,483; 3,143,421; 3,145,105; 3,145,242; 3,360,448; 3,376,208; 3,475,193; and 3,582,339; in British patent specification Nos. 778,365 and 804,005; in Belgian Pat. No. 663,578 and in Japanese Patent Publication No. 24,270/1972.
As a surface treatment-free undercoating method, there is the so-called multi-layer method which comprises providing, as a first layer, a layer containing a resin that is adhesive to polyesters and has low chemical resistance and coating thereon a hydrophilic resin layer as a second layer, as well as the so-called mono-layer method comprising coating only one layer of resin which comprises both a hydrophobic group and a hydrophilic group. However, the latter method has the disadvantages that insufficient adhesive strength is obtained.
Organic solvents have often been used to improve adhesiveness, but are not preferred, because of the difficulty of handling the same, high production costs and, in general, their toxicity. Even in the case of using organic solvents, sufficient adhesiveness between a polyester and a photographic emulsion layer is not easily obtained.
It has further been proposed to add a polyester swelling agent to an undercoating composition so as to activate the polyester, but, according to this method, a large amount of polyester swelling agent must be used in order to obtain sufficient adhesive force. Since such a swelling agent is generally a high boiling point compound and the use thereof in large amounts requires high temperatures and long drying times, swelling of the polyester occurs along with a lowering of elasticity at the initial stages and shrinkage at the later stages of drying, resulting in a lowering of surface quality of the support and wrinkling.
On the other hand, in one method an aqueous undercoating composition is used which is free of organic solvents, i.e., an emulsified copolymer or a water soluble compound is used. For example, one method comprises using an undercoating composition containing an emulsified hydroxymethylated N-(1,1-dimethyl-3-oxobutyl)acrylamide copolymer as disclosed in Japanese Patent Application No. 37,652/1974, an undercoating composition containing an emulsified copolymer of methyl acrylate, vinylidene chloride and itaconic acid as disclosed in U.S. Pat. No. 2,852,378, an undercoating composition containing a polyamide-epichlorohydrin resin as disclosed in Japanese Patent Application No. 74,006/1974 and an undercoating composition containing a polyvalent metal salt such as a stannous salt, stannic salt, ferric salt, chromic salt or zirconium salt, as disclosed in U.S. Pat. No. 3,535,147.
In the case of using an emulsified copolymer, however, various problems are encountered due to the fact that a hydrophobic polymer is dispersed using a dispersion agent, for example, gels tend to occur, filtration must be carried out in several stages, the stability or storage capability of the coating composition is insufficient and various additives used for the dispersion have a harmful influence on photographic emulsion layers applied to the undercoating layer.